Silver-studded Blues on the Purbeck Heaths

Male Silver-studded Blue - Scotland Heath, Dorset

Britain's resident butterflies can be divided into two main groups - the wider countryside species and the habitat specialists. You will see the first group in gardens, parks and farmland - Large White, Meadow Brown and the like. Many are very mobile, and they can turn up pretty much anywhere. Most are familiar to us. 

You have to work harder to find the second group - our habitat specialists. These are restricted to certain special places, for example where their foodplants grow, and they are less likely to travel over the wider landscape. You usually have to make an effort to find them!

The Silver-studded Blue Plebejus argus is a classic habitat specialist. In the UK it is mainly found on lowland heaths and certain parts of the English and Welsh coasts (see the distribution map on Butterfly Conservation's helpful fact sheet). But this doesn't tell the whole story. The odd fact about Silver-studded Blues is that they live in two contrasting habitats, where the plant species are very different.

We see this in Dorset, south-west England. Here, most Silver-studded Blues are found on lowland heaths in the Poole Harbour basin area, particularly in the Purbeck area south of Wareham. These heathlands are famously the home of a range of rare and often endangered species - ranging from the charismatic Sand Lizard (see my blogs here and here) to the more obscure Purbeck Mason Wasp (see my blog here).

Silver-studded Blue habitat - Scotland Heath, Purbeck, Dorset

However, you can also come across Silver-studded Blues on Dorset's Isle of Portland, where the lime-rich grassland looks very different to Purbeck's heather and gorse-dominated heathlands. There is a clear gap between the two populations, and there is little (actually, no) evidence of the butterflies moving between the two areas. So, what's going on here? To answer the question we need to look at the butterfly and its unusual ecology.

Introducing the Silver-studded Blue

Although the blue butterflies (part of the Lycaenidae family) are some of Britain's more eye-catching insects, telling the various species apart is not always straightforward - even in Britain with its relatively low butterfly species numbers. Silver-studded Blue males are perhaps most easily confused with Common Blues, which are much more, well, common. As well as being smaller, the most obvious difference is that male Silver-studded Blues have a wider black margin on the wings than male Common Blues (see the photos below).

Male Silver-studded Blue - Grange Heath

For comparison: male Common Blue - Powerstock Common

At which point, you might be asking: what about the 'silver studs'? These are actually on the back of the wings, and they are not always very obvious. Look for the small bright dots in the middle of the black spots - one is arrowed in the image below: 

Male Silver-studded Blue - wing underside. 'Silver studs' arrowed.

The 'silver studs' are also present on the underside of the female's wings, which is just as well because the females - which have dark brown wings - are otherwise tricky to separate from the females of Common Blue, Brown Argus and a few others. The studs can (just about) be seen in this image of a female below (there is a better image later):

Female Silver-studded Blue - Scotland Heath

Luckily, Silver-studded Blues are not difficult to find, as long as you are in the right place. Over the last few weeks I have been carrying out wildlife surveys on the Purbeck Heaths (not for butterflies, as it happens) and Silver-studded Blues have been one of the most numerous and conspicuous insects that we have encountered. They have a characteristic flight pattern, keeping low over clumps of heather, pausing often to rest on flowers. They usually won't fly far from where you first see them; some research says that they rarely move more than 10-20 metres, although from what I've observed this may be an underestimate.  

An unusual Life Cycle

Silver-studded Blue caterpillars (larvae) have two types of foodplants, depending on their habitat. At the Dorset heaths, they feed most commonly on heathers (Calluna vulgaris and species of Erica), while on Portland's limestone grasslands it is more likely to be Bird's-foot Trefoil, Common Rock-rose or Horseshoe Vetch. So far, so different.

What links the two habitats is the relationship between the caterpillars and ants - specifically black ants. Caterpillars produce sugar-rich liquids that attract and feed the ants; in return, the ants look after the caterpillars. It seems that the ants move the caterpillars into underground chambers, from where they emerge at dusk to feed (possibly carried by the ants). Eventually, the butterfly caterpillar will pupate underground, and an adult will emerge in turn. This all seems very benign, and I can't find any evidence of more sinister behaviour taking place, such as caterpillars feeding on the ant brood, as happens with the (even rarer) Large Blue

The ant-caterpillar relationship doesn't happen by chance. Adult female Silver-studded Blues can detect chemical signals (pheromones) produced by the ants, which they will follow to find suitable a site for egg laying. On Purbeck's Grange Heath a couple of weeks ago we were lucky to find a Silver-studded Blue surrounded by black ants:

Silver-studded Blue male with black ants

Nearby, a pair of Silver-studded Blues were mating: the 'silver' studs on the female (more easily seen here) are almost blue in colour. 

Silver-studded Blues mating - male above, female below

While the association with black ants takes place in both habitats, the species of ant differs. On acid heathland it's usually Lasius niger, while on limestone grassland it will be Lasius alienus. I don't know enough about ants to distinguish them in the field, but I presume that the ones photographed above on Grange Heath were L. niger. (If they aren't, please let me know!)

Conserving Silver-studded Blues

In Britain, the combination of recent population decline and restricted distribution of Silver-studded Blues mean that they are classed as 'vulnerable' on the UK Red List, although the species is much more widely distributed in continental Europe. The good news is that even in England and Wales they are usually present in good numbers on the sites where they occur. Even better, numbers have risen dramatically on sites in Purbeck such as Studland, although the picture is less encouraging for the grassland population on Portland (Warren et al., 2025).

On heathland, conservation management aims to maintain a patchwork, or mosaic, of different ages and types of heather as well as some bare ground. Suitable grazing and/or burning can achieve this. For grassland, it is important to manage grazing levels so they do not become too excessive; creation of bare ground areas is also recommended. Such actions will benefit other heathland species too.  

Finding Silver-studded Blues

The photos of Silver-studded Blues in this blog post were all taken on the Purbeck Heaths, which is where (in my experience) the butterflies are most easily found in Dorset. While I have seen them on Portland, numbers are lower than at Purbeck, and there is also a greater risk of confusing them with other blue butterflies. Chalk-hill Blue and the beautiful Adonis Blue are present on Portland's limestone grassland, but absent from Purbeck's acid heaths. The flight period runs from mid-June to the end of August, so now's the time to get out and look for them.  

Many of Purbeck's heathlands are accessible to the public, and most lie within the Purbeck Heaths National Nature Reserve (link here). At RSPB Arne (where I volunteer), Silver-studded Blues can be seen from the trails on Coombe and Hyde's Heaths.

 

Male Silver-studded Blue - Grange Heath

References

Asher, Jim, Warren, Martin, Fox, Richard, Harding, Paul, Jeffcoate, Gail and Jeffcoate, Stephen (2001). The Millennium Atlas of Butterflies in Britain and Ireland. Oxford: OUP.

Hetherington, Mike, Sterling, Phil and Coulthard, Emma (2022). 'Butterfly colonisation of a new chalkland road cutting.' Insect Conservation and Diversity, 15(2), pp.191-199.

Munns, Lorraine (2017). Is it all about the ants? What are the factors influencing the presence of Plebejus argus (the Silver-studded Blue butterfly) on Studland Peninsula? (Doctoral dissertation, Bournemouth University).

Sterling, Phil and Henwood, Barry (2020). Field guide to the caterpillars of Great Britain and Ireland. London: Bloomsbury.

Thomas, C.D. (1993). 'The silver-studded blue, Plebejus argus L.' The conservation biology of Lycaenidae (butterflies), pp.97-99. - link

Thomas, Jeremy, Surry, Richard, Shreeves, Bill and Steel, Carolyn (1998) New Atlas of Dorset Butterflies. DNHAS.

Tolman, Tom and Lewington, Richard (2008) Collins Butterfly Guide. London: HarperCollins. 

Warren, M.S., Rich, S. and Haywood, N. (2025). 'The dramatic rise of the Silver-studded Blue, Plebejus argus, over 50 years at Studland Heath National Nature Reserve, Dorset, UK' - link

Text and images (c) Mike Hetherington

Purbeck Mason Wasps - a star of Dorset's heathlands

 

Purbeck Mason Wasp Pseudepipona herrichii - Grange Heath, Dorset

One of the joys of surveying for reptiles on the Dorset heathlands is the likelihood of being distracted by other exciting wildlife. And this morning on Grange Heath, in the growing heat of June, there was much of interest to distract us - mostly of the insect variety. 

Star billing went to the Purbeck Mason Wasps Pseudepipona herrichii (see image above). Within the British Isles, this handsome hymenopteran has an incredibly restricted range. Essentially, it is only found on the lowland heaths around Poole harbour in the east of Dorset, especially in what is generally known as the 'Isle of Purbeck'. (Spoiler alert - it's not even slightly an island). I've added a link to NBN Atlas distribution map for the species here - the pattern is pretty stark.

Purbeck Mason Wasps are one of the UK's rarest invertebrates, which has been recognised by law: they are a 'priority species' for conservation in the Natural Environment and Rural Communities (NERC) Act 2006 (section 41). A 'species action plan' was published in 1999 (link here) describing actions and activities aimed at improving the wasp's conservation status. This has been taken forward by the 'Back from the Brink' project, with a useful summary sheets here and here.

Grange Heath lies in a quiet, and rather isolated, corner of the wider Purbeck Heaths National Nature Reserve (more details about the overall reserve are here). The land is managed by the RSPB, who make no secret of the presence of Purbeck Mason Wasps on the site:

Grange Heath entrance sign

The heath is managed for several species of conservation interest, including birds, reptiles and invertebrates. Populations are monitored, and it was reptile monitoring that took us to the site this morning. Various actions have been carried out on the heath to benefit Purbeck Mason Wasps, but to fully understand these we need to take a quick look at the ecology of this - frankly rather fussy - species. 

Like many insects, these solitary wasps have a bizarrely specialised life cycle, which probably accounts for their rarity. After winter, males are the first to emerge; they have a short life span, existing only to mate with the females before dying after about 10 days. Not much of a life, you might say. After that, the females do all of the work - and their first job is to dig a burrow, as seen in a short iPhone video from this morning:

Although classed as 'solitary wasps', individual females seem to prefer nesting in groups - although this may be due to the relative scarcity of suitable sites for burrowing. The wasps need open ground that has a high clay content: if it is too sandy then this risks the burrows caving in. The holes are neat and round, with the burrows heading vertically down. The spoil from digging is usually moved slightly away from the hole, so it's worth looking for a small pile of soil granules in the vicinity. The photo below, from this morning at Grange Heath, shows Purbeck Mason Wasps at the entrances of two burrows plus a third burrow (arrowed): 

Purbeck Mason Wasp burrows - two occupied and one empty (arrowed)

At Grange Heath, a number of artificial scrapes have been excavated in suitable clayey substrates - and this has obviously worked. Almost all of the Purbeck Mason Wasps that we saw were buzzing around these cleared areas - and we saw well over 20 individuals.

However, finding suitable ground conditions is only part of the challenge for a female Purbeck Mason Wasp. What makes the wasps unique (you might say 'picky') is their feeding strategy. This has two parts. The adult females seek out a high energy food source in the form of nectar. However, only one species of plant will do: Bell Heather Erica cinerea. The wasps' tongues aren't long enough to reach into the tubular openings of these richly purple-coloured flowers, so they have to bite through the base of the flower (the calyx) to obtain nectar. The presence of Bell Heather is therefore another habitat requirement for the species. Fortunately, there's plenty of this on Grange Heath.

The second part of the feeding equation involves provisioning the wasps' offspring, and (once again) only one type of food will do. This time it's the caterpillars (larvae) of a small heathland moth variously known at Heath Tortrix, Heath Button Moth or (more scientifically) Acleris hyemana. The unfortunate larvae are caught by the female wasps, brought back to base and stuffed into the newly dug burrow. An egg is then laid on top of a ready-made food source for the growing wasp larva. Grisly, but not unusual in the insect world.

But it doesn't always go to plan for the wasp. The female will usually leave the burrow unsealed in her search for several moth larvae to feed her future brood. This provides an opportunity for Black Ants Formica fusca to nip in and steal the hapless moth larvae, which will then be taken back to the ants' nest for a similarly unpleasant end. We were fortunate this morning to see a caterpillar in the process of being moved by some ants from one of the many nearby wasp burrows (below). 

Acleris hyemana larva being moved by Black Ants

Any strategy aimed at conserving the wasps must therefore also seek to conserve the moths. While these have a wider distribution than that of the wasps (NBN distribution map link here), they are not always common in heathland environments, showing a clear preference for shorter and younger heather. Managed burns of small areas of heathland can achieve this, as happens at Grange Heath. 

The story of the Purbeck Mason Wasps provides an insight into both the complexity of heathland ecosystems and the challenges of conserving what are often rare species in the British context. But it's surely worth it in order to maintain the interest and diversity of Dorset's fabulous lowland heaths.

Purbeck Mason Wasp entering burrow - Grange Heath, Dorset

Acknowledgements

 

Particular thanks to my sharp-eyed fellow reptile surveyors (Howard, Chris, Sue and Rob) for their help in locating wasps, burrows, larvae and ants at Grange Heath this morning, and to all the RSPB Dorset Reserves team involved in protecting and managing this fabulous heathland site.

Thanks also to Phil Sterling for confirming identification of the Acleris hyemana larva.